fsverity: Fix silent truncation in bpf_get_fsverity_digest()

bpf_get_fsverity_digest() silently truncates the digest if the provided
buffer is too small.  This is a footgun, and it doesn't match the
semantics of the equivalent UAPI (FS_IOC_MEASURE_VERITY).

Change it to return -EOVERFLOW instead, matching FS_IOC_MEASURE_VERITY.

Fixes: 67814c00de31 ("bpf, fsverity: Add kfunc bpf_get_fsverity_digest")
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Acked-by: Song Liu <song@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/bpf/20260803181232.14743-3-ebiggers@kernel.org
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

diff --git a/fs/verity/measure.c b/fs/verity/measure.c
index f8b3526af00469b7bc20236198fdb44c1915bbbd..68dfccb69772010b15e4ae02bc47f4f422b0af27 100644 (file)
--- a/fs/verity/measure.c
+++ b/fs/verity/measure.c
@@ -144,14 +144,15 @@ __bpf_kfunc int bpf_get_fsverity_digest(struct file *file, const struct bpf_dynp
 
        hash_alg = vi->tree_params.hash_alg;
 
+       out_digest_sz = dynptr_sz - sizeof(struct fsverity_digest);
+       if (out_digest_sz < hash_alg->digest_size)
+               return -EOVERFLOW;
+
        arg->digest_algorithm = hash_alg - fsverity_hash_algs;
        arg->digest_size = hash_alg->digest_size;
 
-       out_digest_sz = dynptr_sz - sizeof(struct fsverity_digest);
-
        /* copy digest */
-       memcpy(arg->digest, vi->file_digest,
-              min(hash_alg->digest_size, out_digest_sz));
+       memcpy(arg->digest, vi->file_digest, hash_alg->digest_size);
 
        /* fill the extra buffer with zeros */
        if (out_digest_sz > hash_alg->digest_size)